This application is based on Patent Application Nos. 2001-293834 and 2001-293835 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a discharge lamp with improved light distribution characteristics.
2. Description of Related Art
Examples of conventional discharge lamps include a lamp using a double helical arc tube in which a discharge path with a double helix configuration is formed and electrodes are provided at both ends of the discharge path (first conventional technique), and a lamp using an arc tube composed of a plurality of U-shaped tubes (e.g., three U-shaped tubes) whose side edges are connected with one another to form a single tortuous discharge path and electrodes are provided at both ends of the discharge path (second conventional technique).
FIGS. 1 and 2 are for describing the first conventional technique. FIG. 1 is a top view of a discharge lamp 901 relating to the first conventional technique as viewed from the top of an arc tube. FIG. 2 is a side view of the discharge lamp 901. As shown in FIG. 1, a top part of the arc tube of the discharge lamp 901 has the following shape. The arc tube top part is composed of a turning part 906 that is provided at the center of the arc tube top part and helical parts 907 that are provided to sandwich the turning part 906 between them. In the arc tube top part, spaces 908 are formed between the turning part 906 and the respective helical parts 907.
FIGS. 3 and 4 are for describing the second conventional technique. FIG. 3 is a top view of a discharge lamp 920 relating to the second conventional technique as viewed from the top of an arc tube. FIG. 4 is a side view of the discharge lamp 920. The discharge lamp 920 relating to the second conventional technique includes an arc tube composed of three U-shaped tubes 921, 922, and 923. The U-shaped tubes 921 and 922, and the U-shaped tubes 922 and 923 are respectively connected with each other at their side edges, thereby forming a single tortuous discharge path. A pair of electrodes (not shown) is provided at both ends of the discharge path. The discharge lamp 920 further includes an arc tube holder 924 that holds a bottom part of the arc tube. The three U-shaped tubes are set straight and provided annularly so as to surround the axis of the arc tube holder 924.
The discharge lamps described above can be used as alternative light sources to incandescent electric lamps. However, these discharge lamps have problems in their light distribution characteristics. To be more specific, the discharge lamps relating to the above conventional techniques exhibit lower illuminance in the direction of the arc tube top (hereafter referred to as xe2x80x9cvertical illuminancexe2x80x9d) than incandescent lamps.
Light distribution characteristics are to indicate distribution of light outputs. A discharge lamp has a base usually provided at a bottom part of an arc tube that is opposite to an arc tube top part, and is set with the arc tub top part being oriented downward. When used as ceiling area lighting of a room, a discharge lamp with high vertical illuminance can lighten the whole room brightly, but a discharge lamp with low vertical illuminance can lighten the whole room only dimly due to light escaping in the horizontal direction. To sum up, there are increasing demands for improving vertical illuminance of discharge lamps.
Also, various efforts have been made to improve light distribution characteristics of discharge lamps relating to the second conventional technique that include a plurality of connected U-shaped tubes. One example of such efforts is a lamp in which a top part of an arc tube is bent toward an axis of an arc tube holder, and another example is a lamp in which an arc tube is inclined with respect to an arc tube holder (see e.g., Japanese published unexamined patent application No. S58-48349 and Japanese published unexamined utility model application No. H2-97746).
In fact, the arc tube whose top part is bent is difficult to manufacture and is not suitable for mass production because it needs a mold with a complex shape and requires a complicated work of inserting a softened glass tube into the mold in its manufacturing processes. Also, the arc tube that is set inclined inevitably increases the whole lamp size and lacks in compactness because an angle of inclination needs to be set large and a distance between both ends of the arc tube increases accordingly.
The object of the present invention therefore is to provide a discharge lamp with improved light distribution characteristics that is easy to manufacture and suitable for mass production.
The above object can be achieved by a discharge lamp, including: an arc tube holder; and a double helical arc tube whose both ends are held by the arc tube holder, and that includes a turning part at a top thereof and two helical parts, the turning part joining the two helical parts together, wherein a tube diameter of the turning part gradually increases toward a middle of the, turning part, in such a direction that narrows a non light-emitting region formed between the turning part and a neighboring portion of each helical part at the top of the arc tube.
According to the construction of the discharge lamp in which spaces formed at both sides of the turning part can be narrowed by adjusting a width of the turning part, light distribution characteristics can be improved. Also, in manufacturing processes of this discharge lamp, the width of the turning part can be adjusted, for example, by blowing air or the like into a glass tube that is in a softened state. Therefore, a normal metal mold can be used in the manufacturing processes. As a result, this discharge lamp is easy to manufacture and suitable for mass production.
The above object can also be achieved by a discharge lamp, including: an arc tube holder; and an arc tube that includes a plurality of U-shaped tubes placed on the arc tube holder, predetermined ones of the U-shaped tubes being connected together, to form a single discharge path therein, wherein each U-shaped tube has a swelling part at a top thereof that is an opposite side to the arc tube holder, to narrow a non light-emitting region surrounded by tops of the plurality of U-shaped tubes.